The present invention is in the field of cryosurgical probes used for freezing and thereby destroying biological tissues. More specifically, the present invention is useful in the field of cryosurgical probes which are cooled by a closed loop Joule-Thomson refrigeration system.
A Joule-Thomson refrigeration system operates by expanding a high pressure gas through an expansion element which incorporates some sort of a flow restriction. The flow restriction might be a small orifice, a narrow capillary tube, or some other sort of restricted passageway. Typically, the refrigeration system includes a source of high pressure gas, a heat exchanger, an expansion element, a heat transfer element, and various tubes or conduits to conduct the gas from one component to another. The high pressure gas passes through the heat exchanger to lower the gas temperature somewhat, then the gas temperature is further lowered in the expansion element, as isenthalpic expansion occurs. The expanded, cooled gas is exposed to the heat transfer element, where the gas absorbs heat which has been transferred from the environment. The operation of a Joule-Thomson refrigeration system can be severely affected by contaminants in the gas, such as water, oil, or particulates. Any such contaminant can easily block the flow restriction in the expansion element, because the flow restriction is typically very small.
Water and oil are particularly detrimental contaminants, because they will selectively collect at the flow restriction, where the majority of the cooling occurs. As the gas expands and cools, the temperature of entrained water and oil also lowers, resulting in the freezing or solidification of the water and oil. This solidification occurs exactly at the flow restriction, because that is where the cooling actually occurs. Water and oil, at least in trace amounts, are often found in ambient air, and they can consequently be introduced into the refrigeration system if any system joints are broken or any system parts are replaced.
Most Joule-Thomson systems are open loop, meaning that the gas is exhausted to the atmosphere after expansion and heat absorption. The source of the high pressure gas in such a system is usually a high pressure gas cylinder. As use proceeds, the amount of gas in the cylinder is depleted. An open loop system such as this can tolerate a certain amount of contamination, because the contaminants are exhausted from the system to the environment along with the gas, during use. If any contamination is introduced into the system during the replacement of parts, or when system joints are broken for other reasons, the contamination is largely flushed out as the gas is subsequently exhausted.
However, it is possible to operate a closed loop Joule-Thomson system, meaning that the gas is repressurized and circulated after expansion. After expansion in the expansion element, exposure to the heat transfer element, and absorption of heat, the low pressure gas is returned to a compressor which can be used to repressurize the gas. The repressurized gas is then circulated back through the heat exchanger and the expansion element. None of the gas is exhausted from the system. Therefore, any contaminants which enter the system are collected in the system, where they accumulate over a period of time. The level of contamination can eventually build up to a level where solidification of the water and oil will plug the expansion element. A method and apparatus have been developed for operating a micro-miniature mixed-gas Joule-Thomson refrigeration system, as disclosed in U.S. patent application Ser. No. 08/542,123, filed Oct. 10,1995, and U.S. patent application Ser. No. 08/698,044, filed Aug. 15,1996, which are incorporated herein for reference. If such a mixed-gas is used, especially in a miniature or micro-miniature refrigeration system, the introduction of air into the system alters the gas mixture ratios, and it can significantly detract from the cooling performance of the gas mixture.
For these reasons, closed loop Joule-Thomson systems are often permanently sealed, to prevent the introduction of contaminants. Replacement of parts, or other breaking of system joints, is not possible in a permanently sealed system. Some systems use self sealing couplings, which automatically close the system when they are broken apart. This automatic sealing limits the amount of leakage and contamination, but some contamination still occurs. Typically, the couplings used in a closed loop system are threaded fittings which are not designed for repetitive disconnection.
The contamination problem becomes more complicated in a closed loop mixed-gas Joule-Thomson refrigeration system which is used in a surgical device, such as a cryosurgical probe. Such a device will typically have a compressor hooked to the probe, with the probe consisting essentially of a handle, a cannula, and a cold tip. The heat exchanger is typically located in the handle, and the expansion element is typically located in the cold tip. The probe cannula or cold tip must be interchangeable with various shapes, such as flat, cylindrical, or sharp edged, to perform different functions. Further, the cold tip must be capable of being sterilized for use in a surgical application, to allow repeated use of the system on different patients.
Known cryosurgical probes are open loop systems for this reason. In an open loop system, the cannula or cold tip can be removed and sterilized or discarded. Introduction of contaminants into the refrigeration system during removal and replacement of the cannula or cold tip is not a significant problem in an open loop system, since the contaminants can be flushed from the system during exhaust of the gas. Open loop systems are wasteful and expensive to operate, because of the necessity of continually replacing the gas. Also, exhaust of the gas to the environment is not always environmentally safe. Closed loop systems are more economical and environmentally safe. If a known closed loop system were used in a surgical application, removal and replacement of the cannula or cold tip for sterilization purposes would introduce contaminants into the system, ultimately resulting in blockage of the expansion element. A closed loop surgical system could theoretically be provided with self sealing couplings, but contamination would still build up over a period of time. Further, self sealing couplings incorporate O-rings and precision parts. Sterilization of the cannula or cold tip would inevitably expose the O-rings and precision parts to high temperatures and harsh chemicals, ultimately resulting in degradation of the sealing ability of the couplings.
Use of disposable replacement cannulas or cold tips would not solve this dilemma. First, even if the replaceable parts are discarded and replaced with new, sterile parts, repetitive disconnections are required, ultimately resulting in the buildup of contaminants. Second, most disposable parts are constructed of plastic, for reasons of economy. Plastics typically contain trace amounts of water. If a plastic part is exposed to the gas in a refrigeration system, the water can eventually leech out of the plastic and contaminate the gas in the system. Third, self sealing fittings typically add size, weight, and significant cost to a device, making them undesirable for use in a disposable device. Fourth, each time a disposable element, such as a cannula or cold tip, is discarded, the refrigerant gas contained within the disposable element is lost. This requires replacement of the gas to avoid degradation of the cooling performance of the system. Evacuation of gas from the disposable component, or use of replacement components precharged with gas, would significantly add to the complexity and cost of the system.
Further, a typical cryosurgical probe will have one or more auxiliary instruments near the cold tip, for use in conjunction with the cold tip, such as temperature sensors, heaters, ultrasound transducers, optical elements, and fluid ports for irrigation and aspiration. If a reusable probe is employed, repetitive sterilization of these auxiliary instruments can degrade their performance. The ideal practice would be to incorporate these auxiliary instruments into a disposable element.
Finally, it is desirable to insulate the shaft of a cryosurgical probe, to prevent freezing of tissue at undesired sites along the probe when the probe is inserted into a body cavity or organ. One effective means of insulation would be to provide a vacuum space along the probe shaft. However, the level of the vacuum maintained in such a space can degrade over time, because of the outgassing of metals, plastics, and braze joints. This outgassing increases during sterilization procedures in which heat is applied to the probe. Therefore, it would be desirable to incorporate the vacuum insulation space into a disposable element. The disposable element would only be sterilized once, and the disposable element can then be economically discarded, minimizing the amount of vacuum degradation.